Abstract:

The dependence of aboveground biomass and
productivity of tropical forests on soil fertility is not fully
understood, since previous studies yielded contrasting
results. Here, we quantify aboveground biomass (AGB)
and stem wood production, and examine the impact of soil
chemistry on these parameters in mature tropical forest
stands of the equatorial Andes in Ecuador. In 80 plots of
0.04 ha at four elevation levels (500, 1,000, 1,500 and
2,000 m a.s.l., total sample area = 3.2 ha), we measured ten
important soil chemical parameters, inventoried all trees
¸10 cm dbh and monitored stem diameter growth with
dendrometer tapes in 32 plots. Top canopy height and stem
density signiWcantly decreased from 500 to 2,000 m, while
tree basal area increased and AGB remained invariant
(344 § 17 Mg DM ha¡1, mean § SE) with elevation. Wood
speciWc gravity (WSG) showed a signiWcant, but small,
decrease. Stem wood production decreased from 4.5 to
3.2 Mg DM ha¡1 year¡1 along the transect, indicating a
higher biomass turnover at lower elevations. The only soil
variable that covaried with AGB was exchangeable K in the
topsoil. WSG increased with decreases in N mineralisation
rate, soil pH and extractable Ca and P concentrations.
Structural equation modelling (SEM) revealed that nitrogen
availability acts on stem wood production only indirectly
through a negative relation between N mineralisation rate
and WSG, and a positive eVect of a lowered WSG on stem
growth. The SEM analysis showed neither direct nor indirect
eVects of resin-extractable P on wood production, but a
negative P inXuence on AGB. We conclude that nitrogen
availability signiWcantly inXuences productivity in these
Andean forests, but both N and P are aVecting wood production
mainly indirectly through alterations in WSG and
stem density; the growth-promoting eVect of N is apparently
larger than that of P.